For building structures, in particular, high-rise buildings, it is required that H-beam steels with a thickness of 100 mm or more (hereinafter, referred to as ultra-thick H-beam steels) are used. These ultra-thick H-beam steels are required to have high performance such as improved toughness as well as increased strength, for example, in accordance with strict safety standards. Conventionally, a rolled formed steel having large amounts of Cu, Nb. V, and Mo added thereto in order to suppress formation of island martensite is proposed (see, for example, Patent Document 1).
Further, these H-beam steels have specific shapes, and hence, rolling conditions (temperatures and rolling reductions) are limited in universal rolling. Thus, rolling finishing temperatures, rolling reduction, and the rate of cooling are more likely to vary depending on the portions of ultra-thick H-beam steel used, especially in the case of a web, flanges, and fillets. As a result, strength, ductility, and toughness vary depending on portions in the ultra-thick H-beam steel, and some portions of the steel may not satisfy requirements, for example, for the rolled steels for welded structure (JIS G 3106).
In particular, if ultra-thick H-beam steels are manufactured by applying hot rolling to blooms obtained through continuous casting, it is difficult to secure toughness through reduction in the size of crystal grain. This is because the maximum thickness of the bloom that continuous-casting equipment can manufacture is limited, and hence, it is not possible to obtain sufficient rolling reduction during rolling operations. Further, if rolling is performed at high temperatures to obtain products with high dimensional accuracy, the thick flange portion has high rolling temperature, which leads to a decrease in the rate of cooling. As a result, at the flange portion, crystal grains coarsen, and in particular, toughness is more likely to deteriorate.
To address these problems, there is proposed a method of reducing the size of crystal grains by diffusing Ti-based oxide in the steel to generate intragranular ferrite (see, for example, Patent Document 2). Further, there is proposed a method of manufacturing high-strength rolled formed steels exhibiting excellent toughness through temperature-controlled rolling and accelerated cooling in addition to reduction in the size and diffusion of Ti oxide and TiN (see, for example, Patent Documents 3 to 5). Further, a manufacturing method in which the amount of carbon contained is reduced to improve toughness is proposed (for example, Patent Document 6).